Check if an array can be divided into pairs whose sum is divisible by k

Given an array of integers and a number k, write a function that returns true if given array can be divided into pairs such that sum of every pair is divisible by k.

Examples:

Input: arr[] = {9, 7, 5, 3}, k = 6
Output: True
We can divide array into (9, 3) and (7, 5).
Sum of both of these pairs is a multiple of 6.

Input: arr[] = {92, 75, 65, 48, 45, 35}, k = 10
Output: True
We can divide array into (92, 48), (75, 65) and 
(45, 35). Sum of all these pairs is a multiple of 10.

Input: arr[] = {91, 74, 66, 48}, k = 10
Output: False

A:

Use Hashing to 0 to k-1. and # of i == # k-1-i.  并且 if k %2 = 0, # k/2 can also be divided by 2